Low loss contact structures for silicon based optical modulators and methods of manufacture

ABSTRACT

The present invention provides silicon based thin-film structures that can be used to form high frequency optical modulators. Devices of the invention are formed as layered structures that have a thin-film dielectric layer, such as silicon dioxide, sandwiched between silicon layers. In one aspect of the invention an electrical contact structure is provided. The electrical contact structure comprises a connecting portion that electrically connects an active region of at least one of the silicon layers to a contact portion of the electrical contact structure.

This application claims the benefit of U.S. Provisional Application Ser.No. 60/554,457, filed Mar. 18, 2004, entitled “Silicon Based OpticalModulators and Methods of Manufacture,” which disclosure is incorporatedherein by reference in its entirety for all purposes.

TECHNICAL FIELD

The present invention relates to silicon based optical modulators foroptical transmission systems. More particularly, the present inventionrelates to electrical contact structures for silicon based thin-filmstructures for use in optical modulators and methods of making suchstructures.

BACKGROUND

The state of the art in optical communication networks, particularlythat related to photonics based components for use in such networks, hasadvanced rapidly in recent years. Present applications require, andfuture application will demand, that these communication systems havethe capability to reliably transfer large amounts of data at high rates.Moreover, because these networks need to be provided in a cost efficientmanner, especially for “last mile” applications, a great deal of efforthas been directed toward reducing the cost of such photonic componentswhile improving their performance.

Typical optical communications systems use fiber optic cables as thebackbone of the communication system because fiber optics can transmitdata at rates that far exceed the capabilities of wire basedcommunication networks. A typical fiber optic based communicationnetwork uses a transceiver based system that includes various types ofoptoelectronic components. Generally, a transceiver includes a lightsource, means to convert an electrical signal to an optical outputsignal, and means to convert an incoming optical signal back to anelectrical signal. A laser is used to provide the source of light and amodulator is used to turn the light source into an information bearingsignal by controllably turning the light on and off. That is, themodulator converts the light from the laser into a data stream of onesand zeroes that is transmitted by a fiber optic cable. The incomingoptical signal can be converted back to an electrical signal by usingcomponents such as amplifiers and photodetectors to process the signal.

Commercially used optical modulators are either lithium niobate baseddevices or compound semiconductor based devices such as the III-V baseddevices that use gallium arsenide or indium phosphide material systems.Additionally, silicon based devices have been developed. However,silicon based optical modulator technology has not been able to providea device that can perform like the commercially available products andmany problems need to be solved before such silicon based devices cancompete with the commercially available lithium niobate and compoundsemiconductor devices.

Lithium niobate devices rely on an electrooptic effect to provide amodulating function. That is, an electric field is used to change therefractive index of the material through which the light is traveling.These devices are usually provided as a Mach-Zehnder interferometer. Inthis type of modulator, an incoming light source is divided and directedthrough two separate waveguides. An electric field is applied to one ofthe waveguides, which causes the light passing through it to be out ofphase with respect to the light in the other waveguide. When the lightemerges from both waveguides and recombines, it interferesdestructively, effectively turning the light off.

In contrast, compound semiconductor based devices rely on anelectroabsorption effect. In this type of modulator an applied electricfield is also used, but not to vary the refractive index of the materialthrough which the light is propagating. In a compound semiconductormaterial, an electric field can be used to shift the absorption edge ofthe material so that the material becomes opaque to a particularwavelength of light. Therefore, by turning the electric field on andoff, the light can be turned on and off.

One problem with lithium niobate based modulators is that as the datatransfer rate increases for these devices, so must the size of thedevice itself. This requires more material, which can increase cost.These modulator devices are often integrated into packages with othercomponents where the demand for smaller package sizes is continuallyincreasing. Therefore, modulator size is a concern. Another problem withlithium niobate based devices is that the drive voltage can be somewhathigh as compared to compound semiconductor devices. Accordingly, becausea large voltage change between the on and off state is more difficult toproduce than a lower voltage swing, the drive electronics required toprovide such large voltage changes are typically relatively expensiveand can introduce more cost to the systems.

Compound semiconductor modulator devices can be made extremely small andare not limited by the size restrictions of lithium niobate baseddevices. Moreover, these devices can handle high data transfer rates atrelatively low drive voltages. However, current compound semiconductorbased modulators, such as those fabricated from the indium phosphidematerial system, have certain limitations. In particular, these devicescan suffer from problems related to coupling losses and internalabsorption losses, which are generally not present in lithium niobatebased devices.

As an additional concern, the processing and manufacture of compoundsemiconductor based devices is expensive when compared to silicon baseddevices, for example. One reason for this is that many of the basematerials used for compound semiconductor processing are expensive anddifficult to handle. For example, indium phosphide wafers are presentlylimited in size and the largest wafers are expensive. This makes lowcost high volume manufacturing difficult as compared to that which canpotentially be obtained in the manufacture of silicon based devices.

Regarding silicon based technology, a silicon based modulator can bedesigned to function in a manner that is similar to the way a lithiumniobate based device functions in that it changes the phase of the lightpassing through a waveguide. This phase change can be used in aMach-Zehnder type device to form a modulator. More particularly, asilicon based device generally operates on the principle that a regionof high charge concentration can be used to shift the phase of light inthe waveguide. Importantly, the magnitude of the phase shift isproportional to the charge concentration and the length of the chargedregion in a direction in which the light travels. Thus, the ability tocreate a region of sufficient charge density to interact with the lightis essential to be able to induce a phase change, especially one thatcan shift the phase by an amount suitable for use in a Mach-Zehnder typedevice.

In order to provide a charged region that can be used for phaseshifting, these devices are known to use injection of electrons ordepletion of holes in a diode or triode type device. In operation, aconcentration of charge carriers can be provided in an active portion ofa guiding region of a waveguide in these devices. One parameter that isimportant in a silicon based optical modulator is the speed in which acharged region can be created and subsequently dissipated. Moreparticularly, the speed at which charge carriers can be generated aswell as the speed at which charge carries can be removed (byrecombination, for example) affects the speed at which modulation can beperformed. These generation and recombination processes are directlyrelated to the mobility of the charge carriers in the particularmaterial. Because these devices use both single crystal silicon andnon-single crystal silicon and because the mobility of charge carriersin non-single crystal silicon is significantly lower than the mobilityof charge carriers in single crystal silicon, the low mobilitynon-single crystal material unfortunately limits the rate at which thedevice can modulate light.

The present invention thus provides silicon based thin-film structuresthat are capable of rapidly creating and removing a charged region forshifting the phase of light passing through the structure. In accordancewith the present invention, high frequency optical modulators can beformed using silicon-insulator-silicon thin-film structures. In order toprovide a charged region for phase shifting, in accordance with thepresent invention, devices of the present invention are preferablyformed as layered structures that have an insulator layer, such assilicon dioxide, sandwiched between silicon layers. A concentration ofcharge carriers can be provided in a region adjacent to eachsilicon/oxide interface by applying an electrical bias across thesilicon layers. This effectively moves charge carriers from the bulksilicon material toward the oxide layer so they build up in a regionnear the interface.

One parameter that is important in this type of device is the speed inwhich a charged region can be created. This speed is directly related tothe mobility of the charge carriers in the particular material.Therefore, in one embodiment, a high performance device can be providedwhen both silicon layers comprise high-mobility silicon such ascrystalline silicon. High mobility material is particularly preferredfor the active portion of the waveguide of the device. Moreover, tofunction as an optical modulator, such devices preferably includestructure of appropriate materials for rapidly altering the free carrierconcentration across the optical path of a waveguide and preferably thestructure is defined to confine and guide the light through thewaveguide without degrading or attenuating the signal.

Another parameter that is important in this type of device relates tothe efficiency of the device. In particular, losses due to absorption inany part of the device can affect the efficiency of this type of opticaldevice. Optical absorption can occur where metal is used in the devicesuch as at an electrical contact. High efficiency devices are preferredfor many reasons such as for providing devices that can be operated atlow voltages. Accordingly, in one embodiment of the present invention,electrical contact structures are provided wherein contact regions thatuse a metal(s) are spaced apart or away from an active region of thedevice in order to control possible optical losses.

The present invention provides silicon-insulator-silicon structures foroptical modulators having first and second silicon layers with eachpreferably comprising active regions comprising high free carriermobility silicon. Such silicon-insulator-silicon structures aredesirable for high speed optical signal modulation (greater than 1×10⁹hertz, for example). Preferably, silicon that has a bulk free carriermobility of at least 500 centimeters 2/volt-second (cm² V-s) at roomtemperature (if n-type silicon is used) and at least 200 cm²/V-s at roomtemperature (if p-type silicon is used) is provided as starting materialto form a thin-film optical modulator structure in accordance with oneaspect of the present invention. That is, this high mobility silicon maybe provided and further doped to form an active region of a siliconlayer for an optical modulator structure, which doping can change, andtypically lowers, the mobility of the active region from the initialvalue. Preferably, such an active region is doped to a level sufficientto achieve the desired modulation performance. In any case, the dopedactive region is considered to have a high free carrier mobility inaccordance with the present invention if it is formed from a materialthat has a free carrier mobility as set forth above.

It has been estimated that speeds in excess of 1×10⁹ hertz and as highas or greater than 10×10⁹ hertz can be realized when the active regionof the second silicon layer has a mobility that is at least 20%-25% ofthe mobility of the active region of the first silicon layer.Accordingly, the second silicon layer is preferably formed from siliconmaterial that has a mobility that is at least 20%-25% of the mobility ofthe material that is used to form the first silicon layer. To furtherimprove the modulation performance, it is preferable to have the secondlayer mobility at about 50%, and most preferably close to 100%. Thus,the initial silicon material for forming the active region of the secondsilicon layer preferably has a mobility of about 50%, and mostpreferably close to 100% of the initial silicon material for the activeregion of the first silicon layer.

Accordingly, in one aspect of the present invention, a silicon basedthin-film structure for an optical modulator is provided. The thin-filmstructure preferably comprises a waveguide. The waveguide preferablyincludes a guiding region comprising a silicon-insulator-siliconthin-film structure. The silicon-insulator-silicon structure preferablycomprises a dielectric device layer sandwiched between first and secondsilicon device layers. The thin-film structure also preferably comprisesan electrical contact structure for at least one of the first and secondsilicon device layers. The electrical contact structure preferablycomprises a contact portion spaced apart from the guiding region of thewaveguide.

In another aspect of the present invention, a silicon based thin-filmstructure for an optical modulator is provided. The thin-film structuregenerally comprises a substrate, first and second silicon device layers,a thin-film dielectric device layer, a waveguide, and an electricalcontact structure for the first silicon layer. The first silicon devicelayer is preferably formed on the substrate and the thin-film dielectricdevice layer is preferably formed on the first silicon device layer. Thethin-film dielectric device layer preferably has a predeterminedthickness. A second silicon device layer is preferably formed on thethin-film dielectric device layer. The waveguide preferably comprises aguiding region and at least a portion of the first silicon device layer,the thin-film dielectric device layer, and the second silicon devicelayer. The electrical contact structure for the first silicon devicelayer preferably includes a contact portion spaced apart from theguiding region of the waveguide.

In yet another aspect of the present invention, a method of forming asilicon based thin-film structure for an optical modulator is provided.The method generally includes steps of forming asilicon-insulator-silicon thin-film structure, forming a waveguide, andforming an electrical contact structure. Forming thesilicon-insulator-silicon structure preferably comprises providing asubstrate comprising a first silicon layer, depositing a thin-filmdielectric layer on the first silicon layer of the substrate, anddepositing a second silicon layer on the thin-film dielectric layer. Thewaveguide preferably includes a guiding region and at least a portion ofthe first silicon layer, the thin-film dielectric layer, and the secondsilicon layer. The electrical contact structure preferably includes acontact portion spaced apart from the guiding region of the waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

FIG. 1 is a schematic cross-sectional view of an exemplary layeredthin-film silicon-insulator-silicon structure in accordance with thepresent invention that can be used to form an optical modulator;

FIG. 2 is a schematic cross-sectional view of a silicon-on-insulatorsubstrate that can be used to form a layered thin-filmsilicon-insulator-silicon structure in accordance with an embodiment ofthe present invention such as the layered thin-filmsilicon-insulator-silicon structure shown in FIG. 1;

FIG. 3 is a schematic cross-sectional view of the silicon-on-insulatorsubstrate of FIG. 2 showing in particular a thin-film etch stop layerthat has been provided on a silicon layer of the silicon-on-insulatorsubstrate to form a layered structure;

FIG. 4 is a schematic cross-sectional view of the layered structure ofFIG. 3 showing in particular channels that are provided to isolate aportion of the silicon layer of the silicon-on-insulator structure inaccordance with an aspect of the present invention;

FIG. 5 is a schematic cross-sectional view of the layered structure ofFIG. 4 with a thin-film dielectric layer covering the thin-film etchstop layer and filling the channels;

FIG. 6 is a schematic cross-sectional view of the layered structure ofFIG. 5 after partial removal of the covering dielectric layer andremoval of the thin-film etch stop layer, thus leaving a firstelectrically isolated silicon device layer having an exposed surface;

FIG. 7 is a schematic cross-sectional view of the layered structure ofFIG. 6 with a functional thin-film dielectric device layer provided overthe first silicon device layer;

FIG. 8 is a schematic cross-sectional view of the layered structure ofFIG. 7 showing a second silicon device layer overlying part of thethin-film dielectric layer and the first silicon device layer to form athin-film silicon-insulator-silicon structure that, in accordance withthe present invention, can be used to form an optical modulator;

FIG. 9 is a schematic top view of a layered structure that can be usedto form an optical modulator such as the optical modulator shown in FIG.8 with first and second contact structures that can be used to positioncontact regions away from an active region of the layered structure;

FIG. 10 is a schematic cross-sectional side view of the layeredstructure of FIG. 9 showing a thin-film dielectric device layerpositioned between first and second silicon device layers; and

FIG. 11 is a schematic top view of another layered structure that can beused to form an optical modulator such as the optical modulator shown inFIG. 8 with first and second contact structures that can be used toposition contact regions away from an active region of the layeredstructure wherein the first and second contact structures are providedat an angle with respect to a direction of light propagation through thelayered structure.

DETAILED DESCRIPTION

In FIG. 1, a first embodiment of an optical modulator 10 in accordancewith the present invention is schematically illustrated incross-section. As shown, the optical modulator 10 includes a substrate12, preferably silicon, an insulator that preferably comprises buriedoxide layer 14, and a first silicon layer 16. Preferably, the firstsilicon layer 16 and the buried oxide layer 14 are provided as asilicon-on-insulator structure, as conventionally known. However, themodulator 10 does not require use of silicon-on-insulator technology andmay be formed by other techniques including in particular thosedescribed below. Silicon-on-insulator structures are preferred becauseof their compatibility with conventional complementary metal oxidesemiconductor (CMOS) processing. As such, the optical functionality ofan optical modulator (which itself is an electro-optical device) can beintegrated with the electrical functionality of devices such astransistors, resistors, capacitors, and inductors on the same substrate.These electro-optical and electrical devices can be formed by using thecommon processing techniques to provide optical circuits that areintegrated with electrical circuits. Moreover, silicon-on-insulatortechnology provides an easy way to provide a high quality single crystallayer and to electrically isolate plural devices that can be formed inthe silicon layer from each other.

The modulator 10 also preferably includes a thin-film dielectric layer18 sandwiched between the first silicon layer 16 and a second siliconlayer 20. In one preferred embodiment, the thin-film dielectric layer 18comprises a silicon dioxide layer. Also, the first silicon layer 16preferably comprises an electrically isolated layer. That is, thesilicon layer 16 is preferably surrounded by an insulating material inorder to laterally and vertically isolate the silicon layer 16.Preferably, as described in more detail below, a silicon-on-insulatorsubstrate is used and a conventionally known shallow trench isolationprocess can be used to laterally isolate the silicon layer 16 from otheradjacent devices formed on the same substrate. The buried oxide layer ofthe silicon-on-insulator structure can thus provide vertical isolation.As such, the silicon layer 16 (or silicon island) can be structurallyisolated by the thin-film dielectric layer 18 and a surrounding oxidefilled trench 21, which includes portions 22 and 23 that can be seen incross-section.

Although preferred, silicon-on-insulator technology does not need to beused. Any conventionally known or future developed technique capable offunctioning in the same manner to sufficiently isolate device structuresfor forming high frequency optical modulators in accordance with thepresent invention can be used. In particular, such structures generallyrequire sufficient lateral as well as horizontal isolation tofunctionally isolate devices from each other. For example, it iscontemplated that conventionally known techniques such as deep trenchisolation or local oxidation of silicon (LOCOS) can be used to laterallyisolate device structures. Regarding vertical isolation, any techniquethat is conventionally known or future developed for sufficientlyvertically isolating the silicon layer 16 in accordance with the presentinvention may be used.

An oxide layer 24 is also preferably provided, as illustrated, and ispreferably designed in order to at least partially define a waveguide 30that extends for a predetermined distance in a direction of propagationof an electromagnetic field through the waveguide 30. That is, the oxide24 preferably assists to confine light in the waveguide 30. Apropagating electromagnetic field is also referred to as light herein.As shown, the waveguide 30 is preferably at least partially defined bythe thin-film dielectric 18 and the first and second silicon layers 16and 20, respectively.

The waveguide 30 functions to confine and guide light propagatingthrough a guiding region 32 of the waveguide 30. In order to illustratethis guiding and confining functionality, a mode 34 of anelectromagnetic field that can propagate through the guiding region 32of the waveguide 30 is illustrated schematically. More specifically, thewaveguide 30 is preferably designed for single mode transmission. Thatis, the waveguide 30 is preferably designed so that the lowest orderbound mode (also called the fundamental guided mode or trapped mode) canpropagate at the wavelength of interest. For typical opticalcommunications systems, wavelengths in the near infra-red portion of theelectromagnetic spectrum are typically used. For example, wavelengthsaround 1.55 microns are common.

Thin-film structures and techniques for designing such structures foroptical waveguides are well known and any structure capable of confiningand guiding light in accordance with the present invention can be used.For example, such waveguides may include interfaces between thin-filmmaterials having different refractive index, which interfaces can beused in order to guide and confine light in accordance with the presentinvention. Moreover, graded index regions, such as can be formed bycontrollably varying the composition of a material, may be used to guideand confine light as is well known.

The first silicon layer 16, the thin-film dielectric layer 18, and thesecond silicon layer 20 are also preferably designed to be capable ofmodulating light that is traveling through the guiding region 32 of thewaveguide 30. More specifically, at least a portion of the guidingregion 32 of the waveguide 30 is preferably designed to include anactive region. Accordingly, the first silicon layer 16 and the secondsilicon layer 20 are preferably operatively doped to form active regions(or doped regions) in the first and second silicon layers 16 and 20.Preferably, the first and second silicon layers, 16 and 20, are doped ina region or area where it is desired to rapidly alter the free carrierconcentration across the optical path of the light propagating throughthe guiding region 32 of the waveguide 30. As described below, thesilicon layer 16 can be p-type and the silicon layer 20 can be n-type orvice versa. Preferably, the active regions are sufficiently doped inorder to achieve a desired modulation or switching speed.

Also, the modulator 10 preferably includes a first electrical contact 26for providing a contact to the silicon layer 20 and a second electricalcontact 28 for providing a contact to silicon layer 16. As shown thecontacts 26 and 28 are preferably spaced apart from the active portionof the guiding region 32 in order to minimize optical losses that can becaused by metals as are preferably used in such contacts. Low losscontact structures and methods of forming such contacts that can be usedin accordance with the present invention are described in more detailbelow.

Preferably, the contacts 26 and 28 are formed as low resistance ohmiccontacts wherein current varies linearly with applied voltage.Accordingly, the second silicon layer 20 preferably includes a highlydoped region 36 that at least partially forms the contact 26. Forexample, a highly doped region is preferably doped to between 5×10¹⁷/cm³and 2×10¹⁸/cm³. Likewise, the first silicon layer 16 preferably includesa highly doped region 38 that at least partially forms the contact 28.The regions 36 and 38 are preferably doped to correspond with the dopingof the respective silicon layer. That is, if the first silicon layer 16is p-type, the region 38 is also preferably p-type. Similarly, if thesecond silicon layer 20 is n-type, the region 36 is also preferablyn-type. Any contacts capable of providing an electrical bias to theguiding region 30, thereby forming an active region in accordance withthe invention, are contemplated and can be used. Such ohmic contacts arewell known by those in the art of complementary metal oxidesemiconductor (CMOS) processing.

In operation, a phase shift can be produced in light passing through thewaveguide 30 by applying an electrical bias across the structure. Thisbias activates the doped first and second silicon layers, 16 and 20,which thereby causes charge carriers to move toward the dielectric layer18. In particular, contacts 26 and 28 can be used to provide a biasacross the first and second silicon layers, 16 and 20, as the mode 34passes through the guiding region 32 of the waveguide 30. Chargecarriers in the active (doped) regions of the first and second siliconlayers, 16 and 20, move toward the dielectric layer 18 and build up soas to provide regions of charge concentration that together can producea phase shift in the light. This phase shift can be used to modulatelight where the modulator 10 is provided as an arm of a Mach-Zehnderinterferometer, for example.

The first and second silicon layers, 16 and 20, and the dielectric layer18 form an optical device capable of providing a desired phase shift tolight passing through the structure. The structure also functions like acapacitor, at least in the sense that charge carriers move toward thedielectric layer in response to an applied electric field. The structureis preferably not designed for use as a charge storage device (anelectrical capacitor) because such a storage device may require contactmetal positioned near the active portion of the device, which metal cancause optical losses. Moreover, the structure of the present inventionpreferably comprises a sandwich structure comprising active silicon(highly doped) layers rather than a body layer as used in a typicaltransistor.

As noted above, the first silicon layer 16 is preferably formed from alayer of a silicon-on-insulator structure because suchsilicon-on-insulator structures are readily available commercially witha top silicon layer that comprises a single crystal silicon layer, whichcan have a high free carrier mobility. For example, a high free carriermobility is generally greater than at least 500 cm²/V-s at roomtemperature for n-type silicon and at least 200 cm² N-s at roomtemperature for p-type silicon. It is noted that these values for freecarrier mobility represent preferred values for the bulk material (orinitial starting material) that is used to form the first silicon layer16. That is, as described below, the first silicon layer 16 ispreferably formed from an initial silicon material that has a high freecarrier mobility and is subsequently doped to provide a functionaldevice layer or active region for an optical modulator, which doping maychange, and typically lowers, the free carrier mobility of a portion ofthe initial silicon layer. Accordingly, an active region of a devicelayer of an optical modulator in accordance with the present inventionmay have a free carrier mobility that is different from another portionof the same device layer. In any case, it is preferably to start with asilicon material that has a high free carrier mobility, as set forthabove, and form an active region from (or in) that initial material.

In accordance with the present invention, the silicon layer 20 ispreferably provided such that the silicon layer 20 has a free carriermobility that is as close to the free carrier mobility of the siliconlayer 16 as possible (as measured before doping to form active devicelayers for an optical modulator). For example, in one embodiment of anoptical modulator, the silicon layer 20 can be formed as an amorphoussilicon layer that is subsequently crystallized at least partiallywithin the functional optical zone of such layer to provide improvedfree carrier mobility. In another embodiment of the invention, thesilicon layer 20 can be formed with an effective crystal structure byselective epitaxial growth wherein extended lateral overgrowth of suchselective epitaxial growth is used to provide the silicon layer 20 assuch overlies at least some of the thin-film dielectric layer 18.Another technique for providing the silicon layer 20 is by using abonding process whereby any of the necessary layers can be provided asdesired and bonded together to create a desired structure.

Optical modulators in accordance with the present invention, such as theoptical modulator 10 shown in FIG. 1, can be made as described below.Preferably, conventional CMOS processing techniques can be used althoughany other known or developed techniques can be used instead of or incombination. Referring to FIG. 2, a silicon-on-insulator structure 40that includes a substrate 42 (typically silicon), buried oxide layer 44,and first silicon layer 46 is illustrated. Such silicon-on-insulatorsubstrates are commercially available. The thickness of the buried oxidelayer 44 and the thickness of the silicon layer 46 are preferablyselected by considering certain desired properties of the particularoptical modulator to be made, such as the dimensions and/or structure ofthe device, as well as the processing techniques to be used. Also, thesilicon layer 46 preferably has a high free carrier mobility.Preferably, if n-type silicon is used, the silicon layer 46 has a freecarrier mobility of at least 500 cm²/V-s at room temperature. If p-typesilicon is used, the silicon layer 46 preferably has a free carriermobility of at least 200 cm²/V-s at room temperature.

Preferably, a portion of the silicon layer 46 is electrically isolatedto form a first device layer 48 as shown in FIG. 6. Such electricalisolation facilitates the formation of plural devices on the samesubstrate. In the illustrated embodiment, an oxide filled trench 49 isused to define the first device layer 48 by forming a border ofdielectric material around the device perimeter. As shown incross-section, the oxide trench 49 includes oxide portions 50 and 52 aspart of the surrounding trench 49 that isolates the first device layer48 from the remainder of silicon layer 46. The buried oxide layer 44isolates the first device layer 48 from below so that an electricallyisolated island of silicon can be created as the first device layer 48.This can be done, for example, by shallow trench isolation (STI) orlocal oxidation of silicon (LOCOS) procedures as known in conventionalCMOS processing.

In a typical trench isolation process, an etch stop layer 54, (see FIG.3) such as a silicon nitride layer, is first deposited on the siliconlayer 46. Next, conventionally known masking and etching processes areused to form trench 49 as shown in FIG. 4 (trench portions 56 and 58 areshown in cross-section). The buried oxide layer 44, as shown, definesthe depth of the trench 49. Accordingly, the trench 49 preferablydefines the first device layer 48 as an island. An oxide layer 60 isthen provided to fill the trench 49 as shown in FIG. 5, which step alsomay cover the etch stop layer 54, as shown. Next, the oxide layer 60over the first silicon device layer 48 and silicon layer 46 of thestructure shown in FIG. 5 can be planarized, such as by using chemicalmechanical processing (CMP). Then, the etch stop layer 54 (which wouldpreferably be relatively very thin) can be removed, such as by, forexample, an acid bath that can selectively remove the etch stop layer 54from the silicon layer 46 and the first device layer 48. The structureshown in FIG. 6 can thus be provided wherein the first device layer 48is electrically isolated from the silicon layer 46 by the oxide trench49 and the buried oxide layer 44. While shallow trench isolation withsilicon-on-insulator technology is preferred, other techniques asdescribed above, can be used. Moreover, any known or developed methodsfor planarizing or removing materials are contemplated, and suchprocesses may be conducted by any number of combined steps of multiplevarieties.

Preferably, the first device layer 48 is doped to form a p-type activeregion for an optical modulator but the first device layer 48 may bedoped to form an n-type active region if desired. Such doping can bedone before the trench 49 is formed or after the trench is formed.However, such doping is preferably performed in a manner that minimizesthe possibility of undesirable thermal diffusion of dopant species.Dopants such as boron can be used to form p-type regions and dopantssuch as arsenic, phosphorus, and antimony can be used to form n-typeactive regions. Preferably, the first device layer 48 is dopedsufficiently to provide a p-type material suitable for use in opticalmodulation. Conventionally known photolithography and ion implantationprocesses may be used, for example to perform the material doping. It isnoted that such doping may change the free carrier mobility of the firstdevice layer 48 such that it is different from the free carrier mobilityof the initial silicon layer 46. In any case, it is preferred to startwith a high mobility material such as the silicon layer 46 to form thefirst device layer 48. Additionally, as mentioned above, the firstdevice layer is preferably sufficiently doped to form an active regioncapable of achieving the desired modulation performance. For example, bystarting with the silicon layer 46 with the above noted free carriermobility, an active region can be formed that is capable of achievinghigh modulation performance (greater that 1×10⁹ hertz).

Next, as shown in FIG. 7, a thin-film dielectric layer 60 is preferablydeposited over the first device layer 48 and the oxide filled trench 49,as illustrated. The thin-film dielectric layer 60 provides a functionaldevice layer portion over at least part of the first silicon devicelayer 48 for electro-optically creating an optical modulator. Forexample, silicon dioxide having a thickness that is preferably less than100 angstroms may be thermally grown or deposited using conventional lowpressure chemical vapor deposition (LPCVD). Moreover, the thin-filmdielectric layer 48 may be formed from or include other dielectricmaterials, or combination thereof, such as silicon nitride, aluminumoxide, aluminum nitride, as well as those material generallycharacterized as titanates. Any deposition techniques may be used suchas those including chemical vapor deposition, physical vapor deposition,and the like.

After the thin-film dielectric layer 60 is deposited to a desiredfunctional thickness, an amorphous silicon layer (or polycrystallinesilicon layer) is then preferably deposited, patterned, doped, and atleast partially crystallized to form a second silicon device layer 62,and to eventually (after structural processing define a structure suchas shown in FIG. 8 in accordance with an aspect of the presentinvention. In particular at least a portion of second silicon devicelayer 48 that comprises a part of a functional optical modulator (wherethe first and second device layers 48 and 68 overlap with thin-filmdielectric layer 60 in between) is crystallized as an effective singlecrystal structure for increased carrier mobility as described below inmore detail. The second device layer 62 can be provided by any desiredtechnique including deposition, crystallization, and bonding processes.For example, the second device layer 62 can by provided by using adeposition and/or crystallization process as described in commonly ownedco-pending U.S. patent application Ser. No. ______ having AttorneyDocket No. HON0012/US, entitled SILICON-INSULATOR-SILICON THIN-FILMSTRUCTURES FOR OPTICAL MODULATORS AND METHODS OF MANUFACTURE, filed oneven date herewith, the entire disclosure of which is fully incorporatedherein by reference for all purposes. Also, the second device layer 62can be provided by using a bonding process as described in commonlyowned co-pending U.S. patent application Ser. No. ______ having AttorneyDocket No. HON00111/US, entitled BONDED THIN-FILM STRUCTURES FOR OPTICALMODULATORS AND METHODS OF MANUFACTURE, filed on even date herewith, theentire disclosure of which is fully incorporated herein by reference forall purposes. Patterning may be done by any known or developed siliconetching or removal technique to preferably create an island of siliconas the second device layer 62. Doping of the second device layer 62 maybe done in any manner, such as described above with respect to firstdevice layer 48 in order to achieve a desired modulation performance.

Additionally, a cap layer 63, such as silicon dioxide or the like ispreferably formed over the patterned second device layer 62 as shown.The patterning, doping, and crystallizing steps can be performed in anydesired order although the second device layer 62 is preferably at leastpartially crystallized before doping to minimize any potential diffusioneffects of the dopant. Where the first device layer 48 is p-type, thesecond device layer 62 is therefore n-type, and vice versa. Preferably,the second device layer 62 is doped sufficiently to provide an n-typematerial suitable for use in optical modulation.

The silicon material (amorphous or polycrystalline) for the seconddevice layer 62 can be deposited by any technique such as low pressurechemical vapor deposition, for example. The second device layer 62 ispreferably patterned to create a structure wherein a portion of thefirst device layer 48 can be accessed for forming a contact 64 to thefirst device layer 48 such as illustrated. The contact 64 is preferablyan ohmic contact and can be formed by conventionally known techniquesthat may include forming an opening 65 through the cap layer 63 and thedielectric layer 60 to provide access to a surface of the first devicelayer 48. The contact 64 is preferably created at a point sufficientlyspaced from a guiding region 68 to minimize potential absorption relatedloss effects that can be caused by metal materials. The second devicelayer 62 is also preferably patterned to create a structure so that acontact 66 can be provided to the second device layer 62 and such thatthe contact 66 is also sufficiently spaced from the guiding region 68 tominimize potential absorption related loss effects that can be caused bysuch contacts. Structure in accordance with an aspect of the presentinvention that can be used to provide low loss contacts are described infurther detail below. The contact 66 is preferably an ohmic contact andcan be formed by conventionally known techniques that may includeforming an opening 66 in the cap layer 63 to provide access to thesecond device layer 62. Formation of such a contact may also include anydoping and/or deposition steps in order to provide the contact with anydesired electrical properties.

In accordance with an aspect of the present invention, the second devicelayer 62, if provided as an amorphous or polycrystalline material, ispreferably thermally processed such as by using a furnace, epi reactor,rapid thermal processor, heated element, or laser system to at leastpartially crystallize the second device layer 62. Alternatively, thesecond device layer 62 can be provided in accordance with a bondingprocess as described in commonly owned co-pending U.S. patentapplication Ser. No. ______ having Attorney Docket No. HON0011/US,entitled BONDED THIN-FILM STRUCTURES FOR OPTICAL MODULATORS AND METHODSOF MANUFACTURE, filed on even date herewith, the entire disclosure ofwhich is fully incorporated herein by reference for all purposes.Preferably, the second device layer 62 is at least partiallycrystallized so that a crystalline or polycrystalline region withenhanced carrier mobility can be provided in at least an active portionof the guiding region 68. Accordingly, the second silicon layer 62 ispreferably formed from silicon material that has a mobility that is atleast 20%-25% of the mobility of the material that is used to form thefirst device layer 48 (the silicon layer 46). To further improve themodulation performance, it is preferable to have the second siliconlayer 62 mobility at about 50%, and most preferably close to 100%. Thus,the initial silicon material for forming the active region of the secondsilicon layer 62 preferably has a mobility of about 50%, and mostpreferably close to 100% of the initial silicon material for the activeregion of the first silicon layer 48.

Any process can be used that is capable of at least partiallycrystallizing a silicon layer, such as an amorphous silicon layer, toprovide a desired mobility. Such crystallization can be done at any timeafter the second silicon layer is formed. Moreover, any process capableof improving the free carrier mobility of a silicon material, whethercrystalline or not, may be used. Moreover, such a technique can be usedto improve the crystallinity, such as by reducing defects or the like,of a crystalline, polycrystalline or partially crystalline silicon layerfor the purpose of improving free carrier mobility. For example,crystallization of deposited silicon films by furnace, lamp, and lasertechniques at a sufficient temperature and time to achieve a desireddegree of crystallization can be used.

As described above, a waveguide structure that comprises asilicon-insulator-silicon thin-film structure that can guide and confinelight through the waveguide structure can be provided in accordance withthe present invention. Additionally, in accordance with the presentinvention, contact structures for controlling and minimizing losses inthe optical path are provided. Preferably, a contact structure inaccordance with one aspect of the present invention includes a contactportion that is spaced apart from the guiding region of the waveguide.Such a contact structure also preferably includes a connecting portionthat connects an active region of the waveguide to the contact portion.Preferably, as described in more detail below, such a connectingstructure comprises a finger-like structure that extends from theguiding region of the waveguide such as from a guiding edge of a layerof the waveguide. By providing a finger-like structure, the contactportion of the contact structure can be spaced apart from the guidingregion of the waveguide to minimize potential optical losses asdescribed in more detail below. Moreover, using such a finger-likestructure also provides the ability to space the contact portion awayfrom the guiding region of the waveguide without compromising theguiding function of an edge of the waveguide as described in more detailbelow.

One example of a layered silicon-insulator-silicon structure 80 inaccordance with the present invention that can be used for minimizingoptical losses is shown schematically in FIGS. 9 and 10. In FIG. 9 a topschematic view is shown and in FIG. 10 a side cross-sectional schematicview is shown. A waveguide 81 is shown that preferably includes firstlayer 82, thin-film dielectric layer 84, and second layer 86. Thewaveguide 81 is preferably designed so that it can confine and guidelight in a propagation direction, generally indicated by referencenumeral 88 in FIG. 9. The layers 82, 84, and 86 preferably form asilicon-insulator-silicon structure such as those described above. Thatis, the structure 80 can be integrated with the structures describedabove and formed by the same techniques in accordance with the presentinvention.

The layered structure 80 preferably includes a contact structure 83 forthe layer 82 and a contact structure 87 for the layer 86. As shown, thecontact structure 83 includes a contact portion 85 and a connectingportion 92 that connects the contact portion 85 to an active region 89of the layer 82. Also, as shown, the contact structure 87 includes acontact portion 91 and a connecting portion 90 that connects the contactportion 91 to an active region 93 of the layer 86. The contact portions85 and 91 preferably comprise highly doped regions and can be formed inaccordance with the invention as described above.

As shown, the waveguide 81 includes guiding edges that function to guidelight propagating through the waveguide 81 in the propagation direction88. In particular, an edge 95 of the layer 82 and an edge 96 of thelayer 86 work together to provide a guiding function on one side of thewaveguide 81. A guiding function is provided on the other side of theillustrated waveguide 81 by an edge 97 of the layer 82 and an edge 98 ofthe layer 86.

As illustrated, the connecting portions 90 and 92 preferably extendlaterally from the edges 98 and 97, respectively. In particular, theconnecting portions 90 and 92 are preferably designed as finger-likestructures that electrically connect the contact regions 91 and 85 thatare positioned away from the active portion of the waveguide 81 to therespective active regions 93 and 89. By spacing the contact regions 91and 85 away from the active regions, coupling between light passingthough the waveguide 81 and the contact regions 91 and 85 can beminimized thereby minimizing optical losses and providing efficientdevices in accordance with an aspect of the present invention. Becausesuch a finger-like extension interrupts the guiding edge from which itextends, the connecting portions 90 and 92 are preferably designed sothat sufficient guiding is provided by the particular guiding edge. Forexample, referring to the layer 82, the guiding edge 97 is interruptedby the connecting portion 92 of the contact structure 83. In particular,the connecting portion 92 has a width 99 that effectively interrupts theguiding edge 97. Preferably, the width 99 of the connecting portion 92is chosen so that the benefits of spacing the contact region 85 awayfrom the active region 89 of the waveguide 81 can be realized withoutcompromising guiding of light through the waveguide 81. The contactstructure 87 can be designed in the same manner so that both of thecontact regions 91 and 85 can be spaced apart from the active region ofthe waveguide and sufficient guiding within the waveguide 81 can beprovided. That is, the contact structures 83 and 87 are preferablydesigned so that the edges 97 and 98 provide sufficient guiding andconfining of light passing through the waveguide 81.

Preferably, the contact structure 83 is spaced apart along thepropagation direction 88 from the contact structure 87 and at differentlevels, but the contact structures 83 and 87 can overlap if desired.Moreover, any number of contact structures can be used for the layers 82and 86. The contact structures can be positioned on the same side of thewaveguide 81, as shown, or may be positioned on opposite sides of thewaveguide 81. That is, any number of number of contact structuresprovided on one or both sides of the waveguide 81 can be used as long asa desired guiding function can be provided by the waveguide 81. Also,the connecting portions 90 and 92 may follow any desired path to connectthe respective device layer to the contact portions, 91 and 85, and donot need to follow a linear path as is shown in FIG. 9. In any case, acontact structure of the present invention is preferably designed toprovide an electrical connection to an electrically active region of alayer of a silicon-insulator-silicon thin-film structure that controlspotential optical losses by spacing a contact region of the contactstructure away from the active region.

As mentioned, the contact portions 85 and 91 are spaced apart from thewaveguide 81 in order to help to minimize optical losses that can becaused by metal that is often used in the contact portions 85 and 91 ormetal that is used to form conducting lines in an integrated circuit.That is, by spaced apart it is meant that the contact portions 85 and 91do not overlap with any portion of the active regions 89 and 93. Also,the contact portions, 85 and 91, are preferably positioned away from theactive regions, 89 and 93, so that interconnecting circuit lines can berouted to minimize or eliminate optical losses from such lines. In onepreferred embodiment, the contact regions 85 and 91 may be formed about1 micron or more away from the active portion of the guiding region toprevent coupling between a metal in the contact regions 85 and 91 andthe optical signal. However, the contact regions 85 and 91 can be spacedapart from the active portion of the guiding region by any distancesufficient to control losses due to coupling between light in the activeportion of the device and metal in a contact region. Moreover, thecontact regions 85 and 91 can be spaced apart from the active portion ofthe device by the same or different distances.

As illustrated in the FIG. 9 embodiment, the connecting portions 90 and92 extend from the edge 95 at an angle of about 90 degrees (normal tothe propagation direction 88). Also, one or both of the connectingportions 90 and 92 may extend from the edge 95 at an angle less than 90degrees (an oblique angle with respect to the propagation direction 88).For example, in FIG. 11, a layered thin-film structure 100 is shown thatincludes a waveguide 102 formed from a dielectric device layer 104sandwiched between first and second silicon layers 106 and 108. Thethin-film structure 100 may be provided as described above with respectto the thin-film structure 80. As shown, the layered structure 100includes contact structures 110 and 112 that comprise contact regions114 and 116, respectively, as well as connecting portion 118 and 120,respectively. The connecting portions 118 and 120 of the contactstructures 110 and 112 can be designed, as shown, to extend from apropagation direction 122 at an angle that is less than 90 degrees (anoblique angle with respect to the propagation direction). By forming theconnecting portions 118 and 120 at an oblique angle with respect to thepropagation direction 122, it is generally more difficult for lighttraveling in the waveguide to reach any of the contact regions 114 and116, thereby reducing the possibility of optical losses due toabsorption.

The present invention has now been described with reference to severalembodiments thereof. The entire disclosure of any patent or patentapplication identified herein is hereby incorporated by reference. Theforegoing detailed description and examples have been given for clarityof understanding only. No unnecessary limitations are to be understoodtherefrom. It will be apparent to those skilled in the art that manychanges can be made in the embodiments described without departing fromthe scope of the invention. Thus, the scope of the present inventionshould not be limited to the structures described herein, but only bythe structures described by the language of the claims and theequivalents of those structures.

1. A silicon based thin-film structure for an optical modulator, thethin-film structure comprising: a waveguide having a guiding regioncomprising a silicon-insulator-silicon thin-film structure comprising adielectric device layer sandwiched between first and second silicondevice layers; and an electrical contact structure for at least one ofthe first and second silicon device layers, the electrical contactstructure comprising a contact portion and a connecting portion, thecontact portion comprising a metal region spaced apart from the guidingregion of the waveguide and the connecting portion extending from aguiding edge of the at least one of the first and second silicon devicelayers and electrically connecting an active region of the at least oneof the first and second silicon device layers to the contact portion. 2.The thin-film structure of claim 1, wherein the guiding region of thewaveguide is at least partially defined by the active region of thefirst silicon device layer.
 3. The thin-film structure of claim 2,wherein the guiding region of the waveguide is at least partiallydefined by an active region of the second silicon device layer.
 4. Thethin-film structure of claim 1, wherein the connecting portion of theelectrical contact structure extends laterally from a direction ofpropagation of light through the guiding region of the waveguide.
 5. Thethin-film structure of claim 4, wherein the connecting portion extendslaterally from a direction of propagation of light through the guidingregion of the waveguide at an angle normal to the direction ofpropagation of light through the guiding region.
 6. The thin-filmstructure of claim 4, wherein the connecting portion extends laterallyfrom a direction of propagation of light through the guiding portion ofthe waveguide at an oblique angle with respect to the direction ofpropagation of light through the guiding region.
 7. The thin-filmstructure of claim 1, comprising a first electrical contact structurefor the first silicon device layer having a first connecting portionextending from a guiding edge of the first silicon layer and connectedto a first contact portion comprising a metal region spaced apart fromthe guiding region of the waveguide and a second electrical contactstructure for the second silicon device layer having a second connectingportion extending from a guiding edge of the second silicon layer andconnected to a second contact portion comprising a metal region spacedapart from the guiding region of the waveguide.
 8. A silicon basedthin-film structure for an optical modulator, the thin-film structurecomprising: a substrate comprising a first silicon device layer; athin-film dielectric device layer on the first silicon device layer ofthe substrate, the thin-film dielectric device layer having apredetermined thickness to function within a waveguide of an opticalmodulator; a second silicon device layer on the thin-film dielectricdevice layer; a waveguide having a guiding region, the waveguidecomprising at least a portion of the first silicon device layer, thethin-film dielectric device layer, and the second silicon device layer;and an electrical contact structure for the first silicon device layer,the electrical contact structure having a contact portion and aconnecting portion, the contact portion spaced apart from the guidingregion of the waveguide and the connecting portion extending from aguiding edge of the first silicon device layer and electricallyconnecting an active region of the first silicon device layer to thecontact portion.
 9. The thin-film structure of claim 8, wherein theconnecting region of the electrical contact structure extends laterallyfrom a direction of propagation of light through the guiding portion ofthe waveguide.
 10. The thin-film structure of claim 9, wherein theconnecting portion extends laterally from a direction of propagation oflight through the guiding region of the waveguide at an angle normal tothe direction of propagation of light through the guiding region. 11.The thin-film structure of claim 9, wherein the connecting portionextends laterally from a direction of propagation of light through theguiding portion of the waveguide at an oblique angle with respect to thedirection of propagation of light through the guiding region.
 12. Thethin-film structure of claim 8, further comprising a second electricalcontact structure for the second silicon device layer, the secondelectrical contact structure having a contact portion spaced apart fromthe guiding region of the waveguide.
 13. The thin-film structure ofclaim 12, wherein the second electrical contact structure for the secondsilicon device layer comprises a connecting portion that connects anelectrically active region of the second silicon device layer to thecontact portion of the second electrical contact structure.
 14. Thethin-film structure of claim 13, wherein the connecting portion of thesecond electrical contact structure extends laterally from a directionof propagation of light through the guiding region of the waveguide. 15.The thin-film structure of claim 14, wherein the connecting portion ofthe second electrical contact structure extends laterally from adirection of propagation of light through the guiding region of thewaveguide at an angle normal to the direction of propagation of lightthrough the guiding region.
 16. The thin-film structure of claim 14,wherein the connecting portion of the second electrical contactstructure extends laterally from a direction of propagation of lightthrough the guiding portion of the waveguide at an oblique angle withrespect to the direction of propagation of light through the guidingregion.
 17. The thin-film structure of claim 8, wherein the substratecomprises a silicon-on-insulator structure, the silicon-on-insulatorstructure having a first silicon layer and a buried oxide layer.
 18. Thethin-film structure of claim 17, wherein the first silicon device layerof the substrate comprises the silicon layer of the silicon-on-insulatorstructure.
 19. The thin-film structure of claim 8, wherein the firstsilicon device layer comprises a single crystal layer.
 20. The thin-filmstructure of claim 8, wherein the second silicon device layer is atleast partially crystallized in at least a portion of the guiding regionof the waveguide.
 21. The thin-film structure of claim 8, wherein thethin-film dielectric device layer comprises silicon dioxide.
 22. Amethod of making a silicon based thin-film structure for an opticalmodulator, the method comprising the steps of: providing a substratecomprising a first silicon layer; depositing a thin-film dielectriclayer on the first silicon layer of the substrate, the thin-filmdielectric layer having a predetermined thickness; depositing a secondsilicon layer on the thin-film dielectric layer; forming a waveguidehaving a guiding region, the waveguide comprising at least a portion ofthe first silicon layer, the thin-film dielectric layer, and the secondsilicon layer; and forming an electrical contact structure for the firstsilicon layer by providing a contact portion of the contact structure ata predetermined distance from the guiding region of the waveguide andelectrically connecting the contact portion of the contact structure toan active region of the first silicon layer with a connecting portion ofthe contact structure by extending the connecting portion from a guidingedge of the first silicon layer.
 23. The method of claim 22, wherein thestep of forming an electrical contact structure for the first siliconlayer comprises photolithographically patterning the first silicon layerto form the connecting portion that connects the active region of thefirst silicon layer and the contact portion of the electrical contactstructure.
 24. The method of claim 22, further comprising the step offorming an electrical contact structure for the second silicon layer,the electrical contact structure for the second silicon layer having acontact portion comprising a metal region spaced apart from the guidingregion of the waveguide.
 25. The method of claim 22, wherein the step offorming an electrical contact structure for the second silicon layercomprises photolithographically patterning the second silicon layer toform a connecting portion that electrically connects an active portionof the second silicon layer and the contact portion of the electricalcontact structure for the second silicon layer.